1. Field of the Invention
The present invention relates to methods for manufacturing multilayer ceramic electronic elements such as a multilayer ceramic capacitor, and more particularly, relates to an improved method for manufacturing a multilayer ceramic electronic element having a structure in which a ceramic sintered body includes internal electrodes laminated to each other with ceramic layers provided therebetween and in which steps between portions at which the internal electrodes are formed and portions at which the internal electrodes are not formed are smoothed.
2. Description of the Related Art
When multilayer ceramic electronic elements, such as a multilayer ceramic capacitor, are formed, a plurality of ceramic green sheets having internal electrodes printed thereon is laminated to each other. In addition, ceramic green sheets having no internal electrodes are provided at the top and the bottom of the stack of ceramic green sheets thus laminated to form a laminate body. After being compressed in the thickness direction of the laminate body, the laminate body is fired, thereby forming a ceramic sintered body.
When the laminate body described above is compressed, the portion at which the internal electrodes overlap each other and the portion at which the internal electrodes are not formed have thicknesses that are different from each other, and as a result, there has been a problem in that steps are formed between the two portions described above.
In Japanese Unexamined Patent Application Publication No. 8-250370, a method for manufacturing a multilayer ceramic capacitor has been disclosed in which the steps as described above can be smoothed. According to this manufacturing method, ceramic slurry is first printed onto base films by gravure printing to form dielectric ceramic green sheets. Next, conductive paste is applied by gravure printing to the ceramic green sheets thus formed so as to form internal electrodes. Subsequently, ceramic paste for smoothing steps is applied by gravure printing, in a manner similar to the printing mentioned above, to regions in which the internal electrodes are not printed, and the internal electrodes and the ceramic paste for smoothing steps are then dried. By repeating these steps described above at least twice, the structure can be formed in which at least two composite sheets, which are each formed of the internal electrodes and a ceramic pattern for smoothing steps printed onto the base film, are laminated to each other. When a plurality of the composite sheets thus formed is laminated to each other and is then compressed in the thickness direction, a laminate body can be formed, and by firing this laminate body, a ceramic sintered body can be obtained.
According to the technique disclosed in the publication described above, on the surface of the ceramic green sheet, since the ceramic pattern for smoothing steps is printed in regions in which the internal electrodes are not printed, the steps, which are formed between the portion at which the internal electrodes overlap each other and the portion at which the internal electrodes are not formed, can be smoothed.
The miniaturization and higher capacity of multilayer ceramic capacitors have been increasingly advanced, and along with this advancement, attempts to increase the number of layers of the internal electrodes and to decrease the thickness of the ceramic green sheet have been made. As a result, the thickness of the ceramic green sheet is decreased to 3 μm or less, and in some cases, the thickness thereof is even more decreased to 2 μm or less.
In the ceramic green sheet having a small thickness as described above, pin holes may be easily formed or the surface smoothness may be degraded in some cases. In addition, as the number of the internal electrodes is increased, the influence of the steps, which are formed between the portions at which the internal electrodes overlap each other and the portions at which the internal electrodes are not formed, becomes significant.
Accordingly, there has been a problem in that appearance defects of the ceramic sintered body thus formed occur because of the presence of the steps described above, or structural defects, delamination, or other problems occur because of the difference in density between both sides of the step.
According to the technique disclosed in Japanese Unexamined Patent Application Publication No. 8-250370, as described above, an attempt has been made to smooth the steps by printing the ceramic pattern provided for smoothing steps.
However, according to the technique described above, the formation of the ceramic green sheet is performed by gravure printing. In gravure printing, as shown in FIG. 7, a gravure roller 101 having a plurality of printing portions provided on the peripheral surface thereof and aligned in the circumferential direction is used. One printing portion provided on the peripheral surface of the gravure roller 101 is composed of a great number of cells 102, as shown in FIG. 8A.
According to the technique described above, a long base film 103 shown in FIG. 7 is conveyed in the direction indicated by the arrow in the figure, and by using the gravure roller 101 and a roller 104 pressing the base film 103 to the gravure roller 101, printing is performed. For example, when a ceramic green sheet is printed, as shown in FIG. 8A, ceramic slurry 105 is applied to a great number of the cells 102, which form a printing figure, and is then transferred to one major surface of the base film. In this case, as shown in FIG. 8B, the ceramic slurry 105 is dispersed on the base film 103 so as to form a shape that conforms with that of the cells 102. Subsequently, the ceramic slurry 105 dispersed at one position is brought into contact with that dispersed at an adjacent position with time, and hence, as shown in FIG. 8C, leveling is performed, thereby forming a flat ceramic green sheet 106.
However, as described above, according to gravure printing, since the ceramic slurry 105 is transferred from a great number of the cells 102 to the base film 103, and the ceramic green sheet is formed by leveling, there has been a problem in that a time required for forming the ceramic green sheet 106 is determined by the time required for leveling. Accordingly, specifically in the case in which the number of sheets to be laminated to each other is increased, the formation of the ceramic green sheets takes a long period of time, and hence the cost is disadvantageously increased.
In addition, as described above, the ceramic slurry is transferred from a plurality of the cells, and leveling is then performed. However, irregularities tend to be formed on the surface of the ceramic green sheet thus formed, and as a result, the surface smoothness thereof may not be satisfactory in some cases. In addition, variation in thickness of the ceramic green sheet is also inevitably increased. Furthermore, when the thickness of the ceramic green sheet is decreased, a problem may arise in that pinholes are likely to be formed. As a result, for the technique disclosed in the above-described publication, it has been very difficult to respond the trend toward thinner ceramic green sheets.
In addition, after the internal electrodes are formed, the ceramic pattern for smoothing steps is then formed by gravure printing, and next, the internal electrodes and the ceramic pattern are simultaneously dried. Accordingly, exudation is liable to occur between the internal electrodes and the ceramic pattern, and as a result, it has been difficult to improve printing accuracy of the internal electrodes. In particular, after the internal electrodes are printed, exudation is liable to occur along the peripheries of the internal electrodes before the ceramic pattern is printed. Hence, variations in electrostatic capacity of a multilayer capacitor, which is obtained as a product, is inevitably increased.
In addition, according to the technique described in the publication discussed above, after being formed, the ceramic green sheets are in contact with a solvent contained in the conductive paste for forming internal electrode or a solvent contained in the ceramic paste for forming the ceramic pattern for a long period of time. Accordingly, the ceramic green sheet may be damaged by the solvent in some cases, and hence short-circuiting, insulation defect, or other problems are liable to occur in the resulting multilayer capacitor.